The present invention relates to a method and apparatus for simulating a variable voltage attenuator and, more particularly, to a method and a circuit for simulating electrically a slide wire device.
In its simplest form, an electrical slide wire device consists of a length of electrical resistance wire, which is wound on a wire support, and a sliding electrical contact which may be moved to contact electrically any of the turns of the wire along its length. When a reference voltage is applied to the ends of the length of resistance wire, the voltages between the sliding contact and the ends of the wire is directly related to the position of the contact. In effect, the slide wire device becomes a voltage divider or attenuator.
Slide wire devices have been utilized in the past in motor control systems. In a typical system of this type, the motor position is sensed by mechanically linking the sliding contact of a first slide wire device to the motor shaft. When the motor is actuated, the sliding contact is moved correspondingly and the voltage output from the sliding contact provides an indication of the position of the motor shaft. A second slide wire device, also having a reference potential placed thereacross, is utilized to provide a signal related to the desired position of the motor shaft. The outputs from the sliding contacts of the two slide wire devices are then compared by an amplifier/comparator circuit to produce a motor control signal supplied to the motor. As the position of the sliding contact of the second slide wire device is altered, the motor is actuated in such a manner as to move the sliding contact of the first slide wire device to a position in which the outputs from the two sliding contacts are essentially equal. The position of the sliding contact of the second slide wire device may be set manually or, alternatively, may be connected mechanically to a sensor device of some sort which determines the desired position of the motor shaft. Such a conventional prior art slide wire control arrangement is illustrated in U.S. Pat. No. 3,505,606, issued Apr. 7, 1970, to Werner.
A problem develops, however, when it is desired to add an electronic control to a pre-existing control system which utilizes such a pair of slide wire devices. Specifically, it is necessary to provide some arrangement by which a low level signal can be utilized to control the setting of the second slide wire device, so as to provide the signal indicative of the desired position of the motor.
U.S. Pat. No. 3,979,654, issued Sept. 7, 1976, to Guicheteau et al, discloses a circuit which provides for slide wire control of a motor in dependence upon an external control signal. In the Guicheteau et al circuit, a separate motor is driven under control of an external signal to move the sliding contact of the slide wire device to a position such that an appropriate signal output is derived for control of the other motor in the system. It will be appreciated that such a system, including a second motor, is relatively expensive and, further, results in additional complications in circuitry.
A number of electrical circuits have been developed which provide a variable resistance in response to a controlling input signal. Such circuits are shown, for instance, in U.S. Pat. No. 4,015,192, issued Mar. 29, 1977, to Koyanagi, and U.S. Pat. No. 3,803,423, issued Apr. 9, 1974, to Libby. The Libby patent discloses a circuit in FIG. 2 in which a pair of mutually exclusive transistor switching devices are alternately switched ON to connect one or the other of a pair of resistors into the circuit for selected periods of time. The relative connection and disconnection time intervals are controlled as a function of an input signal. The resistance simulated by this switching technique is a resistance intermediate the resistances of the resistors and is dependent upon the percentage of the time during each switching cycle that each of these resistors is connected in the circuit.
The Koyanagi patent discloses a circuit in which various combinations of a plurality of resistors are switched into the circuit to produce any of a plurality of reference voltages. The resistors are essentially connected electrically in parallel. The cyclically changing voltage produced by switching the various combinations of resistors into the circuit is integrated and smoothed. By controlling the duty cycle of the switching pulse signals, the magnitude of the output voltage is controlled. Although the Libby and Koyanagi patents disclose circuits which simulate electrically a variable resistor, the circuits cannot be utilized in place of a slide wire type voltage divider, and therefore may not be incorporated into a slide wire control system to permit electronic control thereof.
It is seen, therefore, that a need exists for a circuit which may be substituted in place of a slide wire device in an existing control system to simulate slide wire operation while permitting control of the simulated slide wire device by a low level signal.